1. Field
The disclosed concept pertains generally to network systems and, more particularly, to such network systems including a network transformer and a network protector.
2. Background Information
Low-voltage electrical power networks consist of interlaced loops or grid systems. These systems are supplied with electrical energy by two or more power sources, in order that the loss of any one power source does not result in an interruption of power. Such systems provide the highest level of reliability possible with conventional power distribution and are normally used to serve high-density load areas. Primary applications include, for example, central or downtown city areas, large buildings, shopping centers, and some industrial plants. These network systems can be of the grid type or spot type with three-phase three-wire or three-phase four-wire service at, for example, 208Y/120V or 480Y/277V.
As shown in FIG. 1, a conventional network system 2 includes a plurality of power sources 4,6,8 and a number of loads 10. Each power source 4,6,8 has a dedicated primary feeder 12,14,16 supplying the network (e.g., through a high voltage switch (not shown)), a network transformer 18,20,22, and a separate and distinct network protector 24,26,28, respectively. For simplicity of disclosure, each power source 4,6,8 is shown as having a single phase, although it is to be understood that each power source can have any number of phases (e.g., without limitation, three phases per primary feeder/network transformer/network protector). Each of the network protectors 24,26,28 includes a circuit breaker (not shown) and a network protector relay (or protective relay) (not shown). Fuses 30,32,34 are electrically connected between the outputs of the respective network protectors 24,26,28 and a network bus 36. The network protectors 24,26,28 and/or the corresponding fuses 30,32,34 isolate the network transformers 18,20,22, respectively, from the network bus 36 (e.g., a low voltage collector bus) when a fault occurs in the corresponding one of the network transformers 18,20,22 or the primary feeders 12,14,16.
Examples of network protector relays are disclosed in U.S. Pat. Nos. 3,947,728; 5,822,165; 5,844,781; 6,504,693; and 6,671,151. Network protector relays trip open a corresponding circuit breaker upon detection of power flow in the reverse direction.
Network transformers have a relatively difficult service and duty requirement. A network transformer typically serves loads that vary from almost zero to a maximum overload, withstands full short circuit conditions without damage to its windings, and generally has a relatively very small space constraint. Network transformers are supplied with a voltage that can range, for example, from about 4 kV to about 34.5 kV (wye or delta).
FIG. 2 shows a conventional three-phase network transformer 50 including three oil-filled chambers 52,54,56, a main tank 58 having an oil level 60, a termination chamber or high voltage terminal compartment 62, a high voltage switch compartment 64 (e.g., including a rotary, non-load breaking air switch 66), three-phase secondary bus work 68, and a mating throat 70 for engagement with a network protector 72 as is shown in FIG. 3. The mating throat 70 includes a flexible shunt 74 for electrical connection to network protector bus work 76 of FIG. 3. Each of the chambers 52,54,56 is separate and independent from the other chambers. Positive pressure can be applied in each of the chambers. The air switch 66 can be interlocked such that the network protector 72 is open, in order to remove the network transformer 50 without de-energizing the primary feeder (e.g., 12 of FIG. 1), and such that it can only be moved to a ground position when the network transformer 50 is de-energized. The network protector 72 also includes a circuit breaker 78 and a network protector relay 80.
Often, the network transformer 50 includes various instruments (not shown). For example, a top oil temperature gauge (not shown) indicates core temperature with white and red pointers. The white pointer indicates the current operating temperature and the red pointer indicates the highest prior or current operating temperature. An oil level gauge (not shown) is calibrated to indicate high, low and 25° C. oil levels.
Known network transformers and network protectors are two separate and distinct components, which are used together as part of a network system. For example, most utilities install a network transformer and a network protector together as a single unit. As shown in FIG. 3, a gasket 82 is disposed between the mating throat 70 of the network transformer 50 and the network protector 72, which are then bolted together. As such, this increases a dimension of the spot vault (not shown), which accommodates the combined length of both the network transformer 50 and the network protector 72.
In another network system 2′, as shown in FIG. 4, some electric utilities add a separate vacuum interrupter (VI) switch 84,86,88 in 480 V spot vaults for ground fault and fire detection trip and lockout mechanisms in a spot network. Various sensors 90,91,92,93,94,95 (e.g., fire detectors and/or ground fault sensors) are disposed at the network protector outputs to the low voltage collector bus 36. These sensors 90-95 are monitored by a suitable controller 96 (e.g., without limitation, a programmable logic controller (PLC); a suitable processor), which controls the VI switches 84,86,88 upstream of the respective network transformers 18,20,22.
There is room for improvement in network systems including a network transformer and a network protector.